Nanotechnology in manufacturing paints for eco-efficient buildings - Nanotechnology in Eco-efficient Construction - page 350

incomplete degradation of NO to nitrate (Ohko et al. , 2009), while degrada-
tion of organic pollutants gives rise to the formation of aliphatic com-
pounds, aromatic compounds, carbonyl compounds, alcohols, alkoxyalcohols
and carboxylic acids (Huang and Li, 2011), which may even be more harmful
than the original pollutant. In this sense, the formation of compounds such
as benzaldehyde, phenol, formic acid, acetic acid, hexamethylene, heptane
or benzene as intermediate products during the photocatalytic degradation
of toluene has been observed (Sun et al. , 2010).
Some of these intermediate products (e.g., carboxylic acids) are strongly
adsorbed on the photocatalyst surface (Huang and Li, 2011), and reduce
the number of active catalyst sites in the photocatalytic paint, which can in
critical cases considerably minimize its effi ciency.
The photocatalytic surface can be regenerated by washing the surface
with water or irradiating it with UV light. However, these methodologies
are not easy to apply in the case of indoor usage, and further research will
be needed in order to develop new methodologies to remove adsorbed
intermediate products from indoor photocatalytic paints.

15.4.2 Secondary emissions from paint constituents

Formation of carbonyls while irradiating photocatalytic paints under either
UV or visible light has been repeatedly observed as part of a set of inves-
tigations to assess the decontamination effi ciency of photocatalytically
active paints in the past. The main source of such carbonyl compounds was
assumed to be the photoinduced decomposition of paint binders (Saltham-
mer and Fuhrmann, 2007; Auvinen and Whirtanen, 2008; Pichat, 2010; Geiss
et al. , 2012).
An exhaustive investigation has recently been carried out in order to
evaluate the impact of the different paint components on the formation of
carbonyl compounds when irradiating photocatalytic paints. Thus, both the
photocatalytic paint and each of its individual components was irradiated
in the presence of 5% pure anatase TiO 2 (Geiss et al. , 2012). The compo-
nents evaluated included cohesion agents, super plasticizers, defoaming
agents and redispersable resins.
Figure 15.3 shows the amount of the secondary emission of carbonyl
compounds during the irradiation of the different paint components in the
presence of pure anatase TiO 2 as photocatalyst. As can be observed, for-
maldehyde, acetaldehyde and acetone appear to be the main degradation
compounds for all the paint components evaluated. Lower amounts of
longer chain carbonyl compounds such as propanal, butanal or hexanal
were formed in the initial stages. A radical mechanism based on the
β

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-scission has been proposed for the degradation of longer-chain carbonyl
compounds (Geiss et al. , 2012).